134   THE INTERNAL COMBUSTION ENGINE [chap-.

fig. 39.—Beducing Gear for Crosby
Indicator.
screw adjustments for locking the spring in place. As lias
already been indicated, the to and fro motion of the
paper is obtained from a cord attached at its other end to some
point in the upper part of a swinging lever of which the lowest
point is connected with the engine piston or some part that
moves with it, so that the motion of the engine piston is
reproduced to a convenient
scale. There is also a
Crosby reducing device for
doing this. It is illustrated
in Pig. 39, and its principle
of action is easily seen
therefrom. In this device
the cord at the bottom can
be fastened direct to the
crosshead, or other part at-
tached to the piston, the
cord passing over guide pul-
leys if necessary. It is
better, however, not to have
a longer cord than necessary,
lest its stretching with the
pull put on it should intro-
duce error in the indicator
card. For very accurate
work the cord is some-
times replaced by steel wire.
88. Reflecting Types o£ Indicator.—Although the indicator
is an important instrument in gas engine work, it does not
occupy the position which it holds in steam engine practice
where lower speeds and pressures are met with. A much more
rapidly moving instrument is needed in internal combustion
engines, and such indicators tend to be fragile and are
therefore little used except for special investigations. The
usual form of such indicator makes uye of a beam of light
reflected from a vertical mirror which is caused to tilt as
the gas pressure rises; at the same time the frame in which
the mirror is held is made to move angularly to and fro in
time with the motion 'of the crosshead, thus producing by